1. Field of the Invention
This invention relates to a color-developing ink. More particularly, the present invention relates to a color-developing ink for non-carbon copying paper which ink gives an image by reacting with a colorless electron-donating dye precursor (hereinafter referred to as a color former).
2. Description of the Prior Art
Non-carbon copying papers consisting of a sheet of a color former and a sheet of an electron-accepting solid acid (hereinafter referred to as a color developer) and giving an image by utilizing the color development reaction between the color former and the color developer are already known and in wide use. As the color developer sheet, those on which an aqueous solution of a color developer is coated and dried on their entire surfaces are in general use, however, depending upon applications, those obtained by printing a color developer only on required parts of the surface of a plain paper or of the uncoated surface of a non-carbon upper paper (this printing is called spot printing) are in use. The color-developer-containing composition which is used for the above purpose is called a color-developing ink.
Most of the color-developing inks for spot printing currently in use are those for flexographic type or gravure type printing containing a low boiling organic solvent. Hence, these inks have had safety and hygienic problems associated with the organic solvent.
For solveing the above problems, Japanese Laid-open Patent Application Nos. 68307/1976, 80410/1976, 94308/1976, 89816/1979, 94910/1979, 148606/1979 and 38826/1980, etc. disclose color-developing inks for letter-press type or offset type printing not using any low boiling organic solvent. Future developments of these inks are expected.
Regrettably, these inks have the following drawbacks because the solvents used in the inks mainly for dissolving the color developers have no sufficient characteristics yet.
(1) The density of a developed image is not sufficient and its color fades with time. PA1 (2) The developed image (letters) blurs. PA1 (3) The rubber rolls of a printer swell. PA1 (4) Staining by color development is caused by abnormal breaking of microcapsules coated on an upper paper. PA1 (5) Transfer of an ink between rubber rolls for ink kneading is inferior at the time of printing. PA1 (6) Scumming occurs in offset printing. Thus, color-developing ink satisfactory in all respects is not yet developed.